<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
    margin: 0;
}
.header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 50px;
    background-color: lightblue;
}
.wrapper {
    display: flex;
}
.left {
    width: 200px;
    height: calc(100vh - 50px);
    margin-top: 50px;
    background-color: lightgreen;
}
.right {
    flex-grow: 1;
    height: calc(100vh - 50px);
    margin-top: 50px;
    overflow-y:auto; 
}
    </style>
</head>
    <body>
        <div class="header">Header</div>
<div class="wrapper">
   <div class="left">Left</div>
   <div class="right">
      Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>
      Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right
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      Right<br>Right<br>Right<br>Right<br>Right<br>
      ight<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>
      Right<br>Right<br>Right<br>Right<br>Right<br>
      ight<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>
      Right<br>Right<br>Right<br>Right<br>Right<br>
      ight<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>Right<br>
      Right<br>Right<br>Right<br>Right<br>Right<br>
   </div>
</div>
</body>
</html>